We tested whether the recently introduced measurement of thrombin generation (TG) in whole blood can be used to evaluate the clotting status of patients on vitamin K antagonist (VKA) prophylaxis. The prothrombin time, and hence the International Normalized Ratio (INR), only evaluates the vitamin K dependent factors II, VII and X but not the anticoagulant factors, protein C and S as well as factor IX. In TG all factors play their role and when thrombomodulin (TM) is added the function of proteins C and S is stressed. The thrombotic tendency in congenital protein C resistance proves the importance of this protein C pathway.
To compare the INR in samples from patients under VKA prophylaxis to TG in whole blood and in platelet rich and platelet poor plasma (PRP, PPP) both in the presence and in the absence of TM.
Blood samples were collected from 123 consenting patients on VKA. In two thirds (67%) the indication for prophylaxis was atrial fibrillation. Other indications included prosthetic valves, lung embolisms or thrombosis. The INR was determined in the PPP of the samples and the patients were stratified into 5 groups: INR of 1.0 to 1.5, 1.5 to 2.5, 2.5 to 3.5, 3.5 to 4.5 and higher than 4.5.
Thrombin generation (TG) was measured via Calibrated Automated Thrombinography (CAT) in whole blood and in PRP and PPP, with and without 20 nM added TM. From the TG curve lag time and time to peak were obtained as well as the maximal thrombin concentration (peak) and the area under the curve (endogenous thrombin potential: ETP). Also red and white blood cells and platelets were counted.
With increasing INR values, the ETP and peak height decrease and lag time and time to peak prolong. All TG parameters measured in whole blood were significantly correlated (p-values< 0.01) with the values determined in both PRP and PPP. INR was linearly correlated with lag time and time to peak (p-value< 0.01), whereas for the concentration dependent parameters (ETP and peak height) the correlation with the INR was hyperbolical (p-value< 0.01).
In plasma, 20 nM TM causes a diminution of ETP and peak of 50-60 % in normals and in patients in the INR 1 – 1.5 group. At higher INR values inhibition is between 25 and 40%, independent of the INR value.
In whole blood, on the contrary, the same concentration of TM causes around 30 % of inhibition in normals and in all patients alike.
Whole blood TG data correlate well with INR and reflect more of the coagulation mechanism than the INR does. Like the INR it does not reflect the function of the VKAs on the natural anticoagulant factors, however. In PPP and PRP addition of TM shows that VKA treatment induces TM resistance in patients with an INR value higher than 1.5.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.